There are various industrial applications where cylindrical rolls are used for such things as forming and/or drying sheet material, such as paper, pulp or corrugating medium. One specific application for such rolls is to form corrugated paper which is then bonded to upper and lower paper web to form a corrugated sandwich structure (cardboard). The exterior surface of the roll is made with longitudinally aligned ridges separated by recessed portions or grooves. The interior surface of the roll defines a closed chamber which is pressurized with a condensable heat transfer medium which is generally steam.
In operation pressurized steam is directed through an inlet which is commonly formed at an end wall of the roll with a rotary pressure seal, with the steam being at a temperature and pressure as high as possibly 400.degree. F. and 200 pounds per square inch. As the steam condenses on the interior surface of the cylindrical side wall of the roll it transmits heat through the side wall and thus heats the paper or cardboard which is in contact with the roll side wall. As the steam condenses on the interior surface, the water is removed from the chamber by a siphon pipe or other removal mechanism and discharged through an outlet which can have a rotary seal joint.
A common arrangement for corrugating rolls is for a set of three rolls to be horizontally aligned, one above the other, with the elongate ridge portions of each roll fitting into the matching valley or recessed portions of the other roll. As these rolls are rotated, the paper or web is fed into the region between the rolls to have heat applied thereto and to be formed in a corrugated pattern. As the resulting corrugated sheet moves from the location between the rolls, it is then bonded to upper and lower paper web to form a corrugated sandwich structure.
By way of further background information, various heat transfer media for this type of rolls have been tried in the past, but substantially all cylinders or rolls used for heating, drying or forming pulp or paper are generally heated by steam condensing on the inner surface of the roll that defines a closed pressure chamber. However, there are possible alternatives to using steam, for example, organic vapors such as Dowtherm and special heat transfer oils. The heat transfer coefficient for film type condensation of steam on stationary surfaces ranges from one thousand to three thousand BTU/(hr) (square feet of surface) (.degree.F.) difference in temperature between the steam and the surface being heated). The corresponding range for organic vapors is 200 to 300 and for oils 10 to 30.
Condensation is a constant temperature process, with the temperature depending upon the pressure. Because the internal volume of the roll is large compared with the rate of steam flow, the pressure is constant throughout. Thus, (provided there are no noncondensable gases) the heat leaves the steam at the same temperature at all points throughout the inner surface of the shell, thus, helping to maintain uniform heat transfer and drying at the water surface of the roll.
As the steam condenses on the interior surface of the roll, heat is transferred first from the steam to the condensate film, then through the film to the metal wall that forms the roll. If the steam is super heated, its temperature will drop before it condenses, but condensation will occur at the same temperature as though it had been saturated at the same pressure. Researchers have established that with about 180.degree. F. super heat the rate of heat transfer to a given area is only about three percent more than for saturated steam at the same pressure.
The ideal steam supply and condensate removal system should supply pure steam (no noncondensables) and maintain a thin, uniform condensate film. If noncondensables are present, and if liquid condensate alone is discharged from the cylinder, the noncondensables accumulate. Since the presence of noncondensable gas reduce the heat transfer capacity and uniformity, special consideration should be given to insuring that the noncondensable gases are not allowed to accumulate. This can be accomplished in various ways. For example, by "blowing through" perhaps twenty percent of the steam supply with the condensate, a steam velocity high enough to purge noncondensables from the entire chamber within the roll can usually be achieved.
Certain special problems must be taken into account in applying well known heat transfer data and steam technology to steam heated rolls. Let it be assumed that the roll is stationary, pressurized steam is being fed into the roll, and a certain amount of condensate (liquid water) has formed and rests on the lower part of the interior surface.
As a roll begins to rotate, this tends to move the condensate in the direction of rotation of the roll; inertial forces tend to retard any change in motion of the condensate; centrifugal forces tend to hold the condensate against the inner periphery of the cylinder; and gravity tends to pull the condensate to the bottom of the cylinder. At very low speeds, the gravitational forces cause the condensate to run down the cylindrical side wall in a thin film that forms a puddle at the bottom of the roll. At slightly higher speeds, the viscous forces drag some of the condensate from the puddle part way up the ascending side wall of roll, but it continues to run down to the puddle. As the speed increases still further, the condensate is dragged higher up the interior surface of the side wall, and centrifugal forces hold the condensate to the side wall in the upper quadrant of the ascending side wall. However, gravity still prevails, and the condensate breaks away from the cylinder wall and "cascades" back to the bottom of the dryer.
The rimming condition is achieved when the centrifugal force becomes sufficiently greater than gravity, allowing the condensate to "go over the top". The speed at which this occurs is greatly dependant upon the amount of condensate present in the dryer, a thin layer being rimmed at a slower speed than a thicker layer. However, on the ascending and descending walls of the cylinder, gravity respectively decelerates and accelerates the condensate layer. This results in a condensate layer that is thickest at the top and thinnest at the bottom and in a relative motion of the condensate (with respect to the side wall) best described as "sloshing". At speeds just above the rimming speed, sloshing is considerable. As the speed is increased, the sloshing diminishes, until, at very high speeds, where the gravitational force is overwhelmed by the centrifugal force, sloshing becomes almost negligible.
Fluid flow within the roll has a marked effect on the heat transfer properties of the condensate. Under non-rimming conditions, droplets of condensations can form on the upper portions on the inner roll surface. With dropwise condensation there is no film, and droplets of condensate form and flow in rivulets in the puddle. There is much less resistance to heat transfer from the steam to the metal than with film condensation. The general requirement for dropwise condensation is a non-wettable surface.
Under rimming conditions, heat transfer is governed both by the thickness of the condensate and by fluid flow characteristics. The thinner the layer and more turbulent the flow, the less the resistance to heat transfer. Thickness of the condensate depends on the design, size, location and clearance of the siphon which extracts the condensate from the interior of the roll, roll speed and diameter, condensating rate and differential pressure. Turbulence depends on the condensate thickness and roll speed and diameter. Minimizing the condensate thickness, although resulting in a minimum of turbulence, will result in a lower resistance to, and greater uniformity of, heat transfer.
To illustrate one of the significant problems in operating such steam heated rolls, let us take the example of a paper corrugating operation where a quantity of paper is being fed between a set of two rolls. The steam in the rolls is at a predetermined pressure and temperature, and as indicated above, with the rolls being rotated at a sufficiently high speed, the condensate that has formed will reach a "rimming" condition where the liquid is distributed substantially uniformly (by centrifugal force) against the interior surface of the cylindrical side wall of the roll. In this condition, with the temperature within the roll being substantially uniform throughout and with heat transfer being substantially uniform through all areas of the cylindrical side wall, the temperature of the outside surface of the cylindrical side wall is substantially uniform over the entire outer surface of the side wall.
However, let it now be assumed that it is desired to feed a different size or type of paper sheet through the corrugating rolls. It is necessary to stop the rolls, and it may take approximately five minutes or so (with the rolls being stationary to make the change over to feed the second paper material through the rolls. During this approximate five minute or so changeover time, the condensate (i.e. water) will have accumulated at the bottom part of the roll, and may reach a depth of, for example, 1/4 inch or greater at the lowest point in the interior surface of the roll. Since liquid water is a relatively poor conductor of heat, that portion of the cylindrical wall of the roll that is beneath the liquid water that has accumulated in the bottom of the roll experiences a significant temperature drop in comparison with the other portions of the side wall of the roll (e.g. possibly several 10.degree. F.). This uneven temperature will cause the roll to be distorted out of a perfectly round shape.
Thus, when the rolls are again starting to rotate, with the paper sheet being fed between the rolls, there will be substantial variations of the temperature at the side wall outer surface that engages the paper sheet. The result is that for a period of time (e.g. one to two minutes) until the surface temperature around the entire side wall surface of the roll becomes uniform, disturbing vibration of the roll will occur, the result being that this portion of the product must be discarded or run at a much lower speed. As the rolls continue to rotate and pick up speed, then the "rimming" occurs, and the temperature around the entire side wall again becomes substantially uniform so that the operation can be carried on in a suitable manner.
In addition to the problem noted above of obtaining substantial uniformity of surface temperature along the outside surface of the side wall of the roll, there is also the overall consideration of optimizing the heat transfer from the heat transfer medium (generally steam) within the roll to the outside surface. One avenue which has been explored extensively to accomplish this is to remove the condensate (i.e. liquid water) from the interior of the roll as effectively as possible so that the liquid film that accumulates on the interior surface of the roll during the rimming condition is as thin as possible. However, the overall problem of obtaining proper heat transfer is complex, and certain facets of this will be discussed later in this text.
It is with the above consideration and others in mind that the apparatus and method of the present invention has been developed.